Field of the Invention
The present invention relates to the machining of workpieces made from a composite material. Although particularly suited to the machining of workpieces made from a composite material (and, in particular, those having an organic matrix, also termed OMC materials), the present invention is however not limited to this application and could just as well be employed for the machining of workpieces made from a metallic, plastic or any other material.
Description of the Related Art
It is known that turbo-engines of modern aircraft increasingly comprise parts made from OMC materials (for example compressor retention casings, the fan casing, etc.) which are chosen for their strength and light weight. Indeed, OMC materials make it possible to obtain workpieces of complex shape having small surfaces and dimensions.
As is known, OMC materials are formed with the aid of fibrous preforms having a two-dimensional or three-dimensional structure and an epoxy resin. The fibrous preforms, once produced, are introduced into a mold into which the resin is injected. Heating the mold in an oven then permits the polymerization of the resin and the formation of the OMC materials.
However, OMC materials, whether they have a two-dimensional or a three-dimensional structure, remain difficult to machine and are vulnerable to delamination and to fibers being torn out. Thus, certain machining operations (for example drilling and tapping) are not advised, or are even forbidden, when they break the continuity of the fibers and risk weakening the mechanical strength of the workpieces.
However, in the case of a compressor casing of a turbo-engine, a number of accessories must be mounted on the body of said casing. Holding the accessories requires attachment plates to be installed, these plates being, in the case of a metal casing, held on the body either by drilling and riveting or by tapping and screwing.
As drilling and tapping are not recommended, turbo-engine designers have therefore had to develop solutions either to find a way around the inability of OMC materials to withstand such machining operations, or to compensate for the weakening of the structure of an OMC workpiece after machining.
Thus, a known solution consists in embedding metal inserts when forming the fibrous preform. The inserts are then held by the fibers of the polymerized OMC material and no contraindicated machining operation is then necessary.
However, such a solution is an option only for workpieces made of OMC material having a two-dimensional structure. It is, indeed, very difficult to carry out in the case of large workpieces which are woven in three dimensions and are of complex shape. Integrating metal inserts when weaving the fibers is made difficult, even impossible, as a consequence of the size and volume of the workpieces, and as a consequence of the considerable swelling of the fibers in the preform.
Therefore, in order to compensate for this drawback, and such that the workpieces can be machined (in particular drilled and tapped), it is known to strengthen the OMC workpiece by means of an increase in material close to the holes which are to be drilled. This distributes the forces acting around the drilled—and possibly tapped—holes, which reduces the risk of the workpiece failing once the attachment plates are riveted or screwed.
However, strengthening in this manner by adding material causes a significant increase in mass, which cannot be satisfactory in the context of attempting to reduce the weight of a turbo-engine.
Furthermore, in order to tap the holes drilled in an OMC workpiece, it is current practice to use taps (for example HSS—High Speed Steel—or carbide) which comprise cutting teeth.
However, apart from wearing extremely quickly, the cutting teeth of taps of this type generate significant cutting forces on the OMC materials, which causes, in particular:                deformation of the OMC workpiece;        heating of the resin of the OMC material;        delaminations of the OMC material in the vicinity of the hole; and        breakage of the fibers of the OMC material.        
Such machining defects, amplified with the fineness of the pitch of the tapping, affect the integrity of the OMC material forming the workpiece to be machined. It is therefore impossible to guarantee the material health of the latter.
In other words, tapping a hole in an OMC workpiece, by using taps having cutting teeth, causes a prejudicial weakening of its mechanical structure and thus of its resistance to forces when in use, once mounted.
Moreover, machining tools are known which comprise projecting annular ribs whose outer surface is covered with abrasive grains and whose radial height changes so as to facilitate the cutting of the furrows. It is thereby possible to proceed in a progressive manner for the cutting of the furrows, the ribs of lesser height providing a first stage in the tapping. Nonetheless, these tools, described in Japanese patent JP 10043943, can remove only small volumes of shavings during their passage in the hole to be tapped and it is necessary to carry out multiple passes in order to cut the shape of the threads.